California Democrats struggled to narrow the field in several U.S. House races critical to the party's hope of taking back Congress in the midterm elections.

None of the five candidates in the Orange County district currently held by retiring Republican Rep. Darrell Issa gained enough support to win the party's official endorsement, exacerbating concerns that a crowded field could make it easier for Republicans to hold the seat. Democrats think their chances are strong for that House seat and six others currently held by Republicans because Hillary Clinton carried those districts in the 2016 presidential contest.

California's top-two primary system means the two highest vote-getters in the June primary advance to the general election, regardless of party, potentially allowing two Republicans to make the ballot if Democrats continue to split the vote.

“If we do not unite around the strongest couple of candidates in this race, you end up with five candidates on the June 5th ballot,” Democrat Mike Levin, a candidate to replace Issa, warned in his endorsement pitch. “We will all potentially lose.”

Levin's pitch wasn't strong enough—delegates in the district split the vote among him, Doug Applegate, the 2016 nominee, and Sara Jacobs, a former aide to Barack Obama. Democrats also failed to unite behind a candidate in the race against Republican Rep. Steve Knight, who represents parts of Los Angeles and Ventura counties. But candidates did win more than 60 percent of the vote in races against GOP Reps. Dana Rohrabacher, Tom McClintock and several other candidates. Those endorsements could be ratified Sunday.

The endorsement fights come during the California Democratic Party's annual convention in San Diego, a gathering of more than 3,000 party activists aimed at ginning up excitement for the 2018 contest. Most of the weekend has centered on opposition to President Donald Trump, with party leaders trying to paper over any still lingering divisions from the 2016 presidential contest.

But intra-party competition will continue in races where no endorsements were won. In four races—including the competitive seat now held by retiring Republican Rep. Ed Royce—there was no endorsement vote at all Saturday.

Party officials will continue quietly encouraging lesser-known candidates to consider dropping out without overtly putting their thumbs on the scale.

“Probably one Democrat will pull ahead, but there is tremendous pressure on other Democrats to drop out,” said Thad Kousser, chair of the political science department at the University of California, San Diego.

If some candidates don't begin to drop out, state and national Democratic officials could take more aggressive tactics, like choosing one candidate to support with television ads.

“Working alongside grass-roots activists and the California Democratic Party, the DCCC is keeping all options on the table to ensure that voters have a Democrat on the ballot this November,” said Drew Godinich, a spokesman for the Democratic Congressional Campaign Committee, which focuses on winning the U.S. House.

That's the least desirable option for a party still healing internal divisions exposed in the 2016 presidential election.

“Even if it's rare to see Democrats boxed out of November, all of the churning and behind-the-scenes, closed-door decisions that have to be made in order to prevent it create a problem for people who want voter control of elected officials,” Kousser said.